Blood is composed of several types of specialized cells that perform distinct jobs. Red blood cells, or erythrocytes, are responsible for transporting oxygen from the lungs to all other tissues. White blood cells, also called leukocytes, are a component of the immune system, helping the body fight off infections. Platelets, or thrombocytes, are small cell fragments that initiate the clotting process to control bleeding. The continuous production of these cells is necessary for survival.
Bone Marrow: The Primary Site of Blood Cell Creation
In adults, the generation of new blood cells is almost exclusively confined to the bone marrow. This soft, spongy tissue is found in the central cavities of certain bones. The primary locations for this activity are the pelvis, sternum (breastbone), vertebrae, and the ends of long bones like the femur and humerus. These sites are filled with red marrow, which actively produces blood cells.
The bone marrow is not uniform throughout the skeletal system. Another type, called yellow marrow, consists mostly of fat cells and does not typically produce blood cells. Under circumstances of high demand, such as significant blood loss, yellow marrow can convert into red marrow to increase blood cell formation.
At the heart of blood cell production within the red marrow are hematopoietic stem cells (HSCs). These are undifferentiated cells that can develop into any type of blood cell. Through a process called hematopoiesis, these stem cells divide and mature into red blood cells, white blood cells, or platelets, replacing older ones that are removed from circulation.
Blood Cell Production Before Birth and in Infancy
The location of blood cell creation shifts dramatically throughout development. Long before bone marrow assumes its role, this function is carried out by other organs. In the earliest stages of embryonic development, the primary site for hematopoiesis is the yolk sac, a structure that provides nourishment to the embryo.
As the fetus develops, the responsibility for blood cell production moves to the liver, which becomes the main hematopoietic organ for much of the gestational period. The spleen also contributes significantly to this process. These organs provide the necessary environment for hematopoietic stem cells to proliferate and differentiate.
Toward the end of pregnancy, a gradual transition begins, with the bone marrow starting to take over as the central site of hematopoiesis. This change continues after birth, and by late infancy, the bone marrow has become the primary location for blood cell manufacturing. The liver and spleen cease their hematopoietic activity.
Other Organs That Can Produce Blood Cells
While the bone marrow is the exclusive site of blood cell production in healthy adults, this process can occur elsewhere under specific circumstances. This phenomenon is known as extramedullary hematopoiesis (EMH). EMH is a compensatory response to a failing or insufficient bone marrow.
This reactivation can be triggered by various medical conditions. For example, in disorders like myelofibrosis, the bone marrow is replaced by fibrous scar tissue, which hampers its ability to produce blood cells. Severe forms of anemia can also place a strain on the marrow that it cannot meet alone.
When EMH occurs, the organs responsible for hematopoiesis during fetal development, principally the liver and the spleen, resume their roles. These organs can become enlarged as they work to compensate for the bone marrow’s shortfall. The lymph nodes may also become involved, although less commonly than the liver and spleen.